Shown
at left is the completed assembly of the pressure-time switch designed
for the EK-217B test boxes (above) that is used for development test and
production calibration for the AS-907/HTF-7000 fuel controls. The timer
and power relay circuit is designed onto a printed circuit board that fits
inside the Dwyer 1950-00 pressure switch housing as shown below.

Figure 3 Cross Section Illustration of PC Board Installation

To
illustrate construction overview, Fig 3 shows a cross section of the Dwyer
pressure switch identifying the spacers and how the circuit board mounts on
the MicroSwitch.

The
PC board and its components are shown in Green and the spacer nuts are blue.
The original relay’s 3-hole SPDT contact pinout was
modified by drilling two holes in the PC board to fit the 4-hole DPDT contact
configuration. Two jumpers were added on the PC board to connect the DPDT
contacts in parallel to use both sets. The new power relay physically fits
inside the pressure switch housing and still meets the electrical
requirements.

The
following construction details show pictures of the switch, explain the
modification and installation of the timer circuit board.

The
modification is to cut the nylon shield down flush, to
slightly below the top of the microswitch. On the left side of the
switch is the 3W resistor used to drop voltage down from 110VAC to 24VAC
before rectifying and filtering to power the timer circuit.

Figure 6 Top View of Circuit Board

Figure 7, Bottom View of Circuit Board

This
is the timer circuit board that fits inside the 1950 pressure switch
enclosure. The rectangular hole in the center fits over the raised
portion in the center of the micro switch. The Timer circuit is located
at the top, and a 10A 225VAC electromechanical relay is located on the back side
at the bottom. The bottom view at right shows the 10A 250VAC relay.